EP0187088A1 - Manufacturing process for an ionisation chamber multidetector and multidetector obtained thereby - Google Patents

Abstract

The invention relates to a method for manufacturing a multidetector with ionization chambers and the multidetector obtained by this method. The partitions (22) of the ionization chambers of the multidetector are held relative to one another and transported together by means of a tool over a bath of resin to be polymerized. After the resin has hardened, the detector therefore includes resin bases (2). To contain the resin bath, the invention provides for making a mold cut from a block of solid resin. This allows handling throughout the process without risk of damage. The release is removed.

Description

The invention relates to a method of manufacturing a multidetector. It also relates to the multidetector obtained by this process.

The muttidetectors concerned are essentially multidetectors with ionization chambers. They are of the type used in X-ray CT scanners. A multidetector with ionization chambers comprises a plurality of chambers, adjacent to each other, and separated from each other by metallized partitions forming an electrode: alternatively anode or cathode. All these chambers are filled with an ionizable gas (Xenon for example). When the gas contained in a chamber is excited by X-rays, it ionizes and gives rise to an ionization current between the electrodes located on either side of this chamber. The intensity of the ionization current reveals the intensity of the exciting X-rays. If we know the intensity of an X-ray emitted, we can deduce the density of radiological absorption of an area of a medium crossed by a part of radiation For this we measure, after crossing the medium, the intensity of this part of radiation when it leads to a chamber located directly above this zone. The adjacent arrangements of the muttidetector chambers make it possible to draw up a map of the radiological absorption densities of the adjacent zones of the medium examined.

One of the most important factors of the intensity of the ionization current is the width of a chamber: the distance which separates the two electrodes, or partitions, from this chamber. In order to make the ionization current measurements in all the chambers homogeneous, ionization chambers of equal width should be produced. For this purpose, ceramic supports are used in which grooves adjacent to each other are produced by sawing. The partitions are then slipped one by one into these grooves to determine the chambers. This method comprises an inconvenience: to slide partitions - electrodes in the grooves it is necessary that the grooves are sufficiently surdimens ata ⁱ with respect to thicknesses of the partition walls. This necessary oversizing is annoying for the precision of the installation of the partitions.

In the current state of the art, CT scanners are equipped with a multidetector comprising approximately one thousand ionization chambers. Distributed over approximately 1 meter, and once the thickness of the partitions has been deducted, the width of the chambers is approximately 0.5 mm. If one wants to obtain a good homogeneity of the width of the chambers, this width should not vary by more or less 10%. It follows that the sawing of the grooves must be carried out with an accuracy greater than or equal to 5% of the width of a chamber (twice 5% = 10%, because there are two partitions per chamber). This leads to machining tolerances of the order of 20 microns. These machining tolerances require costly material.

Furthermore, it is impossible to produce large ceramic supports. In general to make a muftidetector about 1 meter long, you have to align end to end three segments of about 300 mm each. The installation of these segments is delicate since the organization system of the rooms continues even when passing from one segment to another. This means that the assembly tolerance of one segment with another must be done with the same precision as that which prevailed for the machining of the grooves. It is common for operators to work with a binocular magnifier to adjust this assembly of the segments. On the other hand. it is impossible to glue these segments one after the other. In fact, bonding exerts forces on the bonded pieces, which means that ultimately there is no control over the spacing of the pieces thus brought together. Finally, finally, the manufacture of partition supports is further complicated by the fact that it is necessary to produce guard rings. These guard rings, which are metal strips arranged orthogonally to the partitions in the support, are used to pick up parasitic ionization currents. In the current state of technology, each segment of the support is itself composed of four elongated elements, joined to each other. Before these four elements are joined, their des is covered with a layer of silver lacquer which serves as a conductive electrode and which acts as a guard ring. The machining of the grooves is undertaken on a segment only after the four elements which constitute it have been joined by gluing. It is necessary to note that the delicate grooving operation must be undertaken then as many times as there are multidetectors to build.

The object of the present invention is to remedy the drawbacks mentioned by eliminating the grooving operation for the manufacture of multidetectors. The installation of the electrode partitions in the invention is carried out all at once, by means of a specialized tool which maintains all the electrodes at the same time. The bases of the partitions bristle one side of this tool. These bases are then immersed, all at the same time, in a bath of resin hardenable by polymerization. As soon as the resin has hardened, the tool is removed: the partitions are held relative to each other. The accuracy of the gap between adjacent partitions is then determined by the structure of the tool used. In this solution the tool is reusable and therefore there is no need to start the delicate grooving operation again. An operation of the same order of precision is undertaken once and for all, it is that which is used to manufacture the tool.

The curable resin bath must be contained in a mold. However, it appeared that at the time of demolding the layer of resin which holds the bases of the partitions breaks. Indeed, this partition support is large while being relatively thin: it is therefore fragile. On the other hand, the differential forces, supported by the resin on the one hand and by the mold on the other hand during the thermal polymerization operation of the resin, do not make it possible to produce monoblock supports. In the invention, this problem is solved by replacing the mold with a base made of epoxy resin or of another similar material (composite for example). The base is then cut from a larger block of epoxy resin The mold then forms an integral part of the support

• - on taille dans un monobloc de résine un socle de même forme que le multidétecteur à réaliser,
• - on arrange toutes les cloisons dans un outil de forme générale adéquate, de telle manière que les embases de ces cloisons en émergent,
• - on répand sur le socle de la résine à polymériser,
• - on approche l'outil près du socle, et on l'y maintient pendant la polymérisation de la résine, pour que les cloisons qui plongent dans la résine soient soudées au socle.

The subject of the present invention is a method of manufacturing a muttidetector with ionization chambers provided with metallized partitions, characterized in that it comprises the following steps:

• - a base of the same shape as the multidetector to be produced is cut from a resin monoblock,
• - all the partitions are arranged in a generally adequate tool, so that the bases of these partitions emerge therefrom,
• - the resin to be polymerized is spread over the base,
• - The tool is approached near the base, and it is maintained there during the polymerization of the resin, so that the partitions which plunge into the resin are welded to the base.

The invention also relates to a multidetector with ionization chambers provided with metallized partitions, characterized in that it comprises at least one resin base to which the partitions are welded with resin.

• - Figures 1 à 7 : les étapes du procédé de fabrication de l'invention;
• - Figures 8 et 9 : une représentation schématique d'un outil pour la mise en oeuvre de l'invention.

The present invention will be better understood on reading the description which follows and on examining the figures which accompany it. This description is given for information only and is in no way limitative of the invention. In the figures, the same references designate the same elements. They show :

• - Figures 1 to 7: the steps of the manufacturing process of the invention;
• - Figures 8 and 9: a schematic representation of a tool for the implementation of the invention.

Figure 1 shows a block 1 of epoxy resin free of air bubbles because based on polymerized resin under pressure. In a preferred manner in the invention, the quality of the resin is chosen with a damping coefficient favorable to microphony. The block 1 is large enough so that a base 2 of width e, of thickness b and the length L can be cut there. Preferably, the thickness of the block 1 will also be h. Pruning can be done preferably by milling. This way of doing things, which results in not subjecting the base 2 to any mechanical stress, is conducive to obtaining bases of all the desired shapes. In particular, the base 2 will have the shape of a portion of crown centered on a center 3. In an example where the multidetector to be manufactured must be used in a CT scanner of medical engineering, the opening angle ₄ of the portion of crown is approximately 41 ° 40. In this example, the length L is approximately 1 meter, the thickness is 5 to 10 mm, and the width is 50 mm. In the application mentioned, provision is made for placing a point source of X-rays at the location of the center 3. This source radiates a fan-shaped beam with an angular opening substantially equal to the angle 4. The source and the multi-detector are provided to be installed on a third generation CT scanner: a rotary CT scanner.

Figures 2 to 4 show a preferred embodiment of the guard rings. Figure 2a is a section along a radial plane 5 of the base 2 shown in perspective in Figure 2b. This FIG. 2a shows the base 2 provided at its base with a layer 6 representing a frame to reinforce the mechanical solidity of this base. In one example, this frame 6 comprises two sheets 7 and 8 of epoxy resin enclosing a honeycomb structure 9. This honeycomb structure is seen in section. It can also be made of epoxy resin. The honeycomb structure is oriented perpendicular to the sheets 7 and 8, it perfectly supports the expansion constraints imposed by the base 12. It supports them all the better since the structure 9 is also made of resin. The frame 6 can be eliminated by choosing for example for block 1 a resin loaded with glass fibers. In the base 2, three longitudinal grooves numbered 10 to 12 are produced. For example, they are spaced from each other by approximately 16 mm and their depth is approximately 1 mm. The tolerance for making these grooves is not very demanding.

The grooves 10 to 12 can be used to set up guard rings. The positioning of these rings is shown in Figures 3. Figure 3b shows a guard ring 13 in the form of a curved chute conforming to the general shape of the detector. The depth of these chutes, in one example, is 8 mm. Figure 3a shows a section along a plane 5 of the base 2 once the guard rings numbered 13 to 16 have been placed there. These guard rings are separated from each other by insulating strips numbered 17 to 19 made of epoxy resin. The strips 17 to 19 have a thickness slightly less than the width of the grooves 10 to 12. They are easily engaged therein. In the preferred embodiment, where the guard rings are in the form of a trough, the heads of these troughs rise above the upper edges of the strips of resin placed on edge in the grooves. In an example this difference in height is worth about 1 mm, its justification will be studied later. Preferably, the troughs are made of hardened copper (for example 0.3 mm thick), which gives them a certain useful rigidity during their installation. To prevent these troughs from sliding over the base 2, they can be fixed to them by pegs such as 20 regularly spaced throughout the trough. The manufacturing tolerance of the trunking is of the same order as that which presides over the production of the grooves 10 to 12. In any case, it allows the trunking to be produced by pressing copper sheets.

To form the base of the multidetector, it is possible, for example, to coat the guard rings. At this stage of the process there are two possible solutions. Or else a layer of resin to be polymerized is deposited on the walls of the chutes: inside and outside of the chutes. Or else the chutes are coated by drowning them in resin to be polymerized. In both cases, the resin is polymerized before proceeding to the next phase. In the second solution, which is the preferred solution, the resin is cut back inside the troughs by milling. Figure 4 summarizes these operations. We can see the base 2 as well as the reminder, in dots, of the old upper level of this base. The polymerized resin now outcrops at a level referenced 21. This level, also called the table, is lower than the tops of the troughs by about 0.5 mm. We can still see, in dotted lines inside each chute, the height to which the resin has been applied before the preferred resizing operation.

With a tool, which will be studied later, all the electrodes of the base thus prepared are approached at the same time. 5 shows in a plane such that 5 a partition 22 approached above the base 2. The partitions 22 are provided with bases or legs such as 23 to 26. These legs are located on two opposite edges of the partitions. Only the legs 23 and 24, located on the same edge are approached from the base 2. These legs are provided at mid-height with holes such as 27. Once the legs (here 23 and 24) are introduced inside the chutes (here 14 and 16) of the base, or possibly beforehand, a resin not yet polymerized is poured up to a height passing substantially between the middle and the upper edges of the holes 27 of the legs and through the level 21 of the table. The electrode 22 shown is an electrode of a given polarity: for example an anode. The two electrodes adjacent to this electrode and which form with it two adjacent ionization chambers are of reverse polarity: for example cathodes. They are not shown, but one is in a deeper plane and the other is in a plane closer to the observer of the figure. They also have two legs. These are provided to engage respectively in the troughs 13 and 15. As all the electrodes are put in place at the same time, it is completely advantageous to pour resin not yet polymerized at the same time in all the troughs.

The legs of the cathode bases also have holes such as 27. The justification for these holes is to set up a system of communicating vases above the resin layer during polymerization. In this way we avoid the rise of resin by capillary action along the partitions. This polymerization resin fills the chutes as indicated again by streaks in FIG. 5. It is flush again at the level marked 21 mentioned previously. It is important that this level 21 is lower than the top of the troughs forming guard rings. The leakage currents which propagate in the resin are essentially surface currents. It is therefore important not to put resins in contact with partition-anodes with resins in contact with partition-cathodes. This is the reason why, in a preferred manner in the invention, the guard rings have the form of chutes whose heads emerge above the level 21.

Once the operations have been carried out, they are all re-undertaken to produce another base adhering to the bases of the opposite edge of the partitions. In FIG. 6, a second base 28 in all points comparable to the base 2 is approached under the muitidetector so that the other bases are glued to it. Note in this figure that the base 2 is already secured to the multidetector. FIG. 6 also represents a high-voltage supply device for the troughs forming guard rings. A generator 29 supplies the chutes 13 to 16 and 30 to 33 at voltages close to the bias, anode or cathode voltage, to which each of the partitions 22 is subjected by its connection 60. In FIG. 6, the tabs 23 to 26 of the partition 22 immerse in the resin contained in the chutes respectively 16, 14, 33 and 31. All these chutes are maintained at the same potential; they are all connected to a single terminal of the generator 29. It is the same for the chutes 15, 13, 32 and 30 which are connected together to another terminal of the generator 29 since the resin which they contain maintains partitions of polarity opposite to that of partition 22.

Although the realization of the guard rings in the form of metal bands only embedded in the bottom of the bases 2 or 28 is sufficient to capture the leakage current in volume in the resin, the arrangement according to which the tops of the chutes perform a partition of the surface of the resin is conducive to correctly capturing the surface leakage currents. It is remarkable that this protuberance of the tops of the troughs cannot be envisaged in a process of the prior art described where, after the layers of silver lacquer have been put in place, the grooving is carried out. This grooving, of course, starts the layers of silver lacquer at the location of the grooves. It therefore does not eliminate the surface leakage currents. In the technique mentioned, the tops of the troughs were located below the base of the legs of the partitions so that they could slide in the grooves. That is to say that these peaks were below level 21 until which the resin is spread so that it grips the partitions. It is noted that the operations consisting in setting up the chutes, in coating them with resin, then in welding there with resin the bases of the electrodes, succeed because the chutes are maintained in a base which accepts the same dilation as 'them: that is to say the resin base of the invention. The chutes in themselves do not cause differential stress. They are fine and moreover are fixed by pins to the base which drives them in its own deformations.

Figure 7 recalls the device which leads to the manufacturing process according to the invention. This device is a multidetector comprising partitions such as 22 joined and held together by two bases respectively 2 and 28 made essentially of epoxy resin. There are also, at the lower right part, peaks such as 34 of spouts which emerge from the resin.

FIG. 8 represents an example of a tool used for holding the partitions before their bases are immersed in the resin to be polymerized contained in the troughs. FIG. 9 shows, in section in a radial direction such as 35, that the tool comprises two jaws 36 and 37 which enclose the partitions. It is noted that the bases of the partitions are protruding beyond the plane of the tool. Both jaws are movable and can be moved towards one another by any means, for example by screwing the two threaded butterflies 38 and 39 about bolt 40 and 4 ₁ which are used to assemble the two jaws. The two bolts are parallel to each other. They impose a collinear displacement of the two mac _h ories. This movement is recalled in FIG. 8 by the arrows 42 and 43. FIG. 8 represents a tool used for producing a curved multidetector. The angle at the center 44 of this multidetector in an example is 41 ° 40. The tool can be perfectly adapted to the realization of a multidetector which would be straight. To hold the partitions 22, each jaw 36 and 37 is provided with notches 45 and ₄ respectively 6. When an action tends to bring the two closer together jaws, the edges, respectively 51 and 52 of each partition, are pressed against the bottoms of the notches.

Claims (12)

1. Method for manufacturing a multidetector with ionization chambers provided with metallized partitions (22), characterized in that it comprises the following steps: - a base (2) of the same shape as the muitidetector to be produced is cut (FIG. 1) in a monoblock (1) of resin; - Arranging (Figure 8) all the partitions in a tool of generally adequate shape so that the bases (23 - 26) of these partitions emerge (Figure 9); - spreading (FIG. 5) on the base of the resin to be polymerized, - The tool is approached near the base and is kept there during the polymerization of the resin so that the partitions which plunge into the resin are welded to the base (Figure 7). 2. Method according to claim 1, characterized in that the base is cut into a portion of the crown (3). 3. Method according to any one of claims 1 or 2, characterized in that after the pruning, - metallized strips (13 - 16), adjacent to each other, are deposited (FIG. 2a - 2b) on the base, in the same shape as the base and intended to act as a guard ring, - And in that it is coated (FIG. 4) by polymerization of these strips with epoxy resin. 4. Method according to claim 3, characterized in that to coat the strips are drowned in resin to be polymerized and in that this resin is cut after polymerization (Figure 4). 5. Method according to claim 3, characterized in that for coating is deposited a layer of resin to be polymerized on the walls of the strips. 6. Method according to claim 3, characterized in that one fixes (20) the strips to the base before coating them. 7. Method according to any one of claims 3 to 6, characterized in that the strips are given a trough shape (Figure 3b) and in that one limits (21) the coating at the bottom of the chutes so that their tops are not coated. 8. Method according to any one of claims 3 to 7, characterized in that before coating is made in the base of the grooves (10 - 12) parallel to the strips and in that raised on edge, in these grooves, sheets (17 - 19) of already polymerized resin. 9. Method according to any one of claims 1 to 8 characterized in that one re-undertakes the same operations to weld other bases of the partitions to another base (28). 10. Method according to any one of claims 1 to 9 characterized in that before arranging the partitions in the tool holes are made (27) in the bases of these partitions. 11. Multidetector with ionization chambers provided with metallized partitions, characterized in that it comprises at least one base (2) in resin to which the partitions (22) are welded with resin (FIG. 7). 12. Multidetector according to claim 11, characterized in that the base comprises rings (13 - 16) guard embedded in the base, the rings being in the form of troughs (Figure 3b) whose vertices (Figure 4) protrude (21 ) from the base.

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